1. Field of the Invention
The present invention is generally related to a laser with a current confining layer and, more particularly, to a vertical cavity surface emitting laser, or VCSEL, with a layer of oxidizable material that is selectively oxidized in order to form an electrically insulative layer with an electrically conductive opening extending therethrough to confine the flow of electrical current through an active region of the laser.
2. Description of the Prior Art
Many different types of semiconductor lasers are known to those skilled in the art. One type of laser is a vertical cavity surface emitting laser, or VCSEL, which emits light in a direction that is generally perpendicular to an upper surface of the laser structure. Lasers of this type comprise multiple layers of semiconductive material. Typically, a substrate is provided at one end of a stack of semiconductive layers. On the substrate, a first mirror stack and a second mirror stack are arranged with a quantum well active region therebetween. On both sides of the active region, graded or ungraded layers can be provided as a spacer between the mirrors. On the second mirror stack, an electrical contact is disposed. Another electrical contact is provided at the opposite end of the stack of layers in contact with the substrate. An electrical current is caused to flow between the two contacts. This electrical current therefore passes through the second mirror stack, a top graded index region, the active region, a bottom graded index region, the first mirror stack and the substrate. Typically, a preselected portion of the active layer is designated as the active region and the electrical current is caused to flow through the active region in order to induce lasing.
In a paper entitled "Progress in Planarized Vertical Cavity Surface Emitting Laser Devices and Arrays" by Morgan, Chirovski, Focht, Guth, Asom, Leibenguth, Robinson, Lee and Jewell, which was published in Volume 1562 of the International Society for Optical Engineering, Devices for Optical Processing (1991), a VCSEL structure is described in detail. The article describes a batch-processed VCSEL comprising gallium arsenide and aluminum gallium arsenide. Several different sizes of devices were studied experimentally and are described in the publication. Continuous-wave threshold currents were measured to 1.7 mA with output powers greater than 3.7 mW at room temperature. The paper also discusses certain interesting characteristics such as differential quantum efficiencies exceeding unity and multitransverse mode behavior. In FIG. 1 of this paper, a perspective sectional view of a VCSEL is illustrated with the various layers identified. In order to confine the current flow through the active region of the quantum well, the device described and illustrated in this paper uses a hydrogen ion implant technique to create electrically insulative regions with an electrically conductive opening extending therethrough. From the upper electrical contact of the VCSEL, current is caused to flow through the electrically conductive opening and thereby is directed through a preselected active region of an active layer.
U.S. Pat. No. 5,245,622, which illustrated to Jewell et al on Sep. 14, 1993, discloses a vertical cavity surface emitting laser with an intra-cavity structure. In the Figures of the Jewell et al patent, a current blocking region is identified by reference numeral 44 and described as forming an annular proton implantation into the active region. The implantation is utilized to horizontally confine the flow of electrical current. The VCSELs disclosed in the Jewell et al patent have various intra-cavity structure to achieve low series resistance, high power efficiencies, and a specific type of modal radiation. In one embodiment of the described VCSEL, the laser comprises a laser cavity disposed between an upper and a lower mirror. The cavity comprises upper and lower spacer layers sandwiching an active region. A stratified electrode for conducting electrical current to the active region comprises the upper mirror and the upper spacer. The stratified electrode comprises a plurality of alternating high and low doped layers for achieving low series resistance without increasing the optical absorption. The VCSEL further comprises a current aperture as a disk shaped region formed in the stratified electrode for suppressing higher mode radiation. The current aperture is formed by reducing or eliminating the conductivity of the annular surrounding regions. In one embodiment, a metal contact layer with an optical aperture is formed on the upper mirror of the VCSEL. The optical aperture blocks the optical field in such a manner that it eliminates higher transverse mode lasing.
U.S. Pat. No. 5,258,990, which issued to Olbright et al on Nov. 2, 1993, describes a visible light surface emitting semiconductor laser. In the Figures of the Olbright et al patent, an active quantum well is identified by reference numeral 34 and is defined by an annular zone identified by reference numeral 33. The annular zone comprises implanted protons which surround the active quantum well and thereby confines the electrical current flow to the quantum well. The VCSEL described in the Olbright et al patent comprises a laser cavity that is sandwiched between two distributed Bragg reflectors. The laser cavity comprises a pair of spacer layers surrounding one or more active, optically emitting quantum well layers having a bandgap in the visible range which serve as the active optically emitting material of the device. Electrical pumping of the laser is achieved by heavily doping the bottom mirror and substrate to one conductivity type and heavily doping regions of the upper mirror with the opposite conductivity type in order to form a diode structure. Furthermore, a suitable voltage is applied to the diode structure. Particular embodiments of the device are described in the olbright et al patent, including those which generate red, green and blue radiation.
U.S. Pat. No. 5,115,442, which issued to Lee et al on May 19, 1992, discloses a vertical cavity surface emitting laser. Lasers of this type are described as depending upon emission through an apertured top surface electrode. Biasing current, accordingly peripheral to the laser as introduced, follows a path which comes to confluence within the active gain region to effectively attain lasing thresholds. The path of the electrical current passes through an opening of a buried region of increasing resistance which encircles the laser at or above the active region. The buried region is produced by ion implantation-induced damage with ion energy magnitude and spectrum chosen to produce an appropriate resistance gradient. Integrated, as well as discrete, lasers are described in this cited reference. The Figures of the Lee et al patent illustrate the location and shape of the region of ion implantation damage which is used to confine the flow of electrical current through an active region within the active layer.
In the April 1993 publication of IEEE Photonics Technology Letters, Vol. 4, No. 4, an article titled "Transverse Mode Control of Vertical-Cavity Top-Surface-Emitting Lasers" by Morgan, Guth, Focht, Asom, Kojima, Rogers and Callis describes transverse mode characteristics and control for VCSELs. The paper discusses a novel spatial filtering concept for the control of VCSEL transverse modes which allow the achievement of over 1.5 mW in certain transverse mode emissions from continuous wave electrically excited VCSELs. This cited paper also illustrates the use of ion implantation for the purpose of current confinement and illustrates a sectional view of this technique in its first figure.
The most commonly known technique for providing the current confinement region of a VCSEL is to use ion bombardment to affect an annularly shaped region and increase its resistance to electrical current. By providing an electrically conductive opening in this region of increased electrical resistance, current is directed through the opening of higher electrical conductivity and can then therefore be directed through a preselected active region within the active layer. It would be beneficial if an alternative method could be provided for achieving current confinement without having to resort to the ion bombardment technique described in the papers and patents cited immediately above.
U.S. Pat. No. 5,373,522, which issued to Holonyak et al on Dec. 13, 1994, discloses a semiconductor device with native aluminum oxide regions. This patent describes a method for forming a native oxide from an aluminum-bearing Group III-V semiconductor material. The method entails exposing the aluminum-bearing Group III-V semiconductor material to a water containing environment and a temperature of at least 375 degrees centigrade to convert at least a portion of the aluminum-bearing material to a native oxide characterized in that the thickness of the native oxide is substantially the same as or less than the thickness of that portion of the aluminum bearing Group III-V semiconductor material thus converted. The native oxide thus formed has particular utility in electrical and optoelectrical devices, such as lasers.
U.S. Pat. No. 5,262,360, which issued to Holonyak et al on Nov. 16, 1993, discloses an AlGaAs native oxide. A method is described for forming a native oxide from an aluminum-bearing Group III-V semiconductor material. It entails exposing the aluminum bearing Group III-V semiconductor material to a water containing environment and a temperature of at least about 375 degrees centigrade to convert at least a portion of the aluminum bearing material to a native oxide characterized in that the thickness of the native oxide is substantially the same as or less than the thickness of that portion of the aluminum bearing Group III-V semiconductor material thus converted. The native oxide thus formed has particular utility in electrical and optoelectrical devices, such as lasers.
U.S. Pat. No. 5,115,442, U.S. Pat. No. 5,245,622, U.S. Pat. No. 5,262,360, U.S. Pat. No. 5,373,522 and U.S. Pat. No. 5,258,990 are hereby expressly incorporated by reference in this description.